OpenGL text stops scrolling and disappears when reaching a specific point

Question

I tried to scroll text horizontally in a OpenGL app. The text comes perfect and smooth part by part at the screen when starting from the right side. When the first letter then touches the side of the window at the left side, the whole text disappears. I want that the text goes over the border, so that it vanished smoothly and start again from the right side. It's a sort of banner i try to make. Someone who knows how to solve this problem?

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <iostream>

#ifdef WIN32
#include <windows.h>
#endif
#include <GL/gl.h>
#include <GL/glut.h>
using namespace std;

//static int font_index = 0;
int state = 1;
float xsize = 800;
float ysize = 300;


void print_bitmap_string(/*void* font,*/ const char* s)
{

    while (*s) {
        glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, *s);
        s++;
    }
}

void reshape(int w, int h)
{
    GLdouble size;
    GLdouble aspect;

    /* Use the whole window. */
    glViewport(0, 0, w, h);

    /* We are going to do some 2-D orthographic drawing. */
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    size = (GLdouble)((w >= h) ? w : h) / 2.0;
    if (w <= h) {
        aspect = (GLdouble)h / (GLdouble)w;
        glOrtho(-size, size, -size * aspect, size * aspect, -100000.0, 100000.0);
    }
    else {
        aspect = (GLdouble)w / (GLdouble)h;
        glOrtho(-size * aspect, size * aspect, -size, size, -100000.0, 100000.0);
    }

    /* Make the world and window coordinates coincide so that 1.0 in */
    /* model space equals one pixel in window space.                 */
    glScaled(aspect, aspect, 1.0);

    /* Now determine where to draw things. */
    //glMatrixMode(GL_MODELVIEW);
    //glLoadIdentity();
}

float yild;
float ystep;
float x_pos = xsize/2;
float y_pos = 70;


void draw()
{
    const char* bitmap_font_names[7] = { "Testing train application for windows!!@@" };

    glPushMatrix();
    /* Draw the strings, according to the current mode and font. */
    glTranslatef(0.5, -100, 0);
    //set the text color
    glColor4f(0.0f, 173.0f, 115.0f, 1.0f);

    //ystep = 100.0;
    //yild = 20.0;
    glRasterPos2f(x_pos, y_pos /* + 1.25 * yild*/);
    print_bitmap_string(bitmap_font_names[0]);
    glPopMatrix();
}

void display(void)
{
    //change background color
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT);
    draw();

    glColor3f(0.0, 255.0, 0.0);
    /*glBegin(GL_TRIANGLES);
    
    glVertex2f(20 + x_pos, 0 + y_pos);
    glVertex2f(50 + x_pos, 10 + y_pos);
    glVertex2f(20 + x_pos, 20 + y_pos);

    glEnd();*/

    glutSwapBuffers();
}


void timer(int) {
    glutPostRedisplay();
    glutTimerFunc(1000 / 90, timer, 0);
    switch (state) {
    case 1:
        if (x_pos > (-xsize / 2) - 200) {
            x_pos -= 1;
        }
        else {
            state = -1;
        }
        break;
    case -1:
        x_pos = (xsize / 2);
        state = 1;
        break;
    }

    cout << x_pos << endl;
}


int main(int argc, char** argv)
{

    glutInitWindowSize(xsize, ysize);
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
    glutCreateWindow("Train Display");
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutTimerFunc(1000, timer, 0);
    glutMainLoop();

    return 0;
}

Answer 1

It looks like OpenGL may not allow you to explicitly draw bitmaps outside of the viewport.

From the documentation on glRasterPos,

To set a valid raster position outside the viewport, first set a valid raster position, then call glBitmap with NULL as the bitmap parameter.

The documentation on glBitmap elaborates on this aspect:

To set a valid raster position outside the viewport, first set a valid raster position inside the viewport, then call glBitmap with NULL as the bitmap parameter and with xmove and ymove set to the offsets of the new raster position. This technique is useful when panning an image around the viewport.